Due to exceptional mechanical properties of diamond, such as its high compressive strength, hardness and wear resistance, existing diamond-coated articles usually fail not due to wear of the coating itself, but due to insufficient strength of the interface between the diamond coating and the substrate.
Diamond coating can be deposited on a metallic substrate or on a substrate made out of ceramic composite material. The examples of metallic substrate may be high-speed steel (HSS) substrate or stainless steel (SS) substrate. Both HSS and SS are alloys of iron (Fe) with carbon (C) and other alloying elements. The examples of the substrate made out of composite material may be cemented carbide substrate, which has a structure of tungsten carbide (WC) grains, bound together by a cobalt (Co) binder.
Diamond coating deposition is carried out at elevated temperatures, usually at 850° C., in the atmosphere of atomic carbon (C) and hydrogen (H). Upon cooling from the deposition temperature to ambient temperature, the dimensions of coated objects usually change and they shrink. The amount of shrinkage is governed by the coefficient of thermal extension (CTE). The CTE's of the diamond coating and the aforementioned substrates differ substantially. For example, HSS and SS substrates have CTE of approximately 13*10−6, and cemented carbide substrates have CTE of 3*10−6, both large compared with the CTE of diamond of 0.3*10−6. Upon cooling from the deposition temperature, due to large difference between the CTE's of the coating and substrate, the substrate shrinks more. Hence, the diamond coating develops high level of compressive residual stress. This CTE mismatch and resulting residual stress buildup limit the thickness of the diamond coating and may result in coating delaminating from the substrate.
It has been established that Fe acts as a catalyst converting the adjacent layer of diamond into graphite. The detrimental catalytic effect of Fe leads to the formation of the layer of graphitic carbon and degrades the adhesion on diamond-cemented carbide interface. Similarly, Co, which is typically used as a binder in cemented carbides, is detrimental to deposition of well-adhered diamond films. In a manner similar to Fe, Co acts as a catalyst converting the adjacent layer of diamond into graphite and degrading the adhesion on diamond-cemented carbide interface. As a result of the described catalytic affect, the direct deposition of diamond on steel or cemented carbide substrate results in the formation of a non-adhering layer of graphitic soot covered by poor-quality diamond. Also, high diffusion rate of carbon atoms into the Fe- or Co-containing substrate leads to loss of carbon atoms from the interface, leaving voids behind and degrading the interface strength even further. As such, the steel and cemented carbide both fall into the category of carbon-sensitive materials.
To improve the adhesion of diamond coatings to carbon-sensitive materials, it has been proposed to use ceramic interfacial layers, acting as diffusion barriers. These ceramic layers are interposed between the substrate and the diamond coating. However, the ceramic interfacial layers do not form chemical bonds with diamond and therefore, do not provide good adhesion between the diamond coating and the substrate. As a result, the adhesion between diamond coating and carbon-sensitive substrates remains a problem.
To improve the adhesion of diamond coating to cemented carbide it has been proposed to etch away the Co binder from the surface layers of cemented carbide. This prevents possible contact of Co with the diamond coating. To this end, various techniques, including acid etching, have been developed. However, etching of Co binder reduces the interfacial surface strength of the cemented carbide article due to loose WC particles. The loose WC particles provide a weak interface for diamond coating. Furthermore, since cemented carbide articles usually fail in a brittle manner, most often due to presence of small cracks at or near the surface, the aforementioned etching of the Co binder substantially reduces the strength of the WC substrate itself.
Binderless cemented carbides with very small fraction of Co binder in them have been suggested, and a diamond coating on them has been proposed. However, the properties of thus made articles have been less than satisfactory due to low fracture toughness of the binderless cemented carbide. It has also been proposed to use thermo-chemical treatment, such as boronizing, i.e., infiltration of the surface layers of cemented carbide with boron, to provide a diffusion barrier separating cobalt binder and diamond coating. However, this leads to a significant loss of fracture toughness of these surface layers and weakening of the cemented carbide article.
The ceramic interfacial layers do not provide stress relieve for diamond coating and do not prevent the residual stress buildup due to CTE mismatch. Thus, upon the cooling to ambient temperature the diamond coating deposited on a substrate with a ceramic interfacial layer is under high residual stress, which limits the coating thickness and may lead to its delamination, as mentioned above.
It has been proposed to use metallic layer of carbide-forming metal to separate the surface of cemented carbide from the diamond coating. However, under the conditions of diamond coating deposition the carbide-forming metal consumes carbon from the surface layers of the cemented carbide, leading to its de-carburization and formation of a brittle η-phase in case of WC. Also, under the conditions of diamond coating deposition the carbide-forming metal may inter-diffuse into the substrate in case of steel.
Also, a two-layered protective coating has been proposed, with the first layer consisting of carbon diffusion barrier selected from the group consisting of MetCO, MetCON and MetON atop and adjacent to the substrate and a second (next innermost) wear resistant coating of at least one layer of MetC, MetN or MetCN, where Met is Ti, Hf, V, Zr, Si, B or other metals of subgroup 3–7 of the periodic table of the elements, or a mixture thereof. However, the oxygen containing in the first layer may diffuse away, resulting in changes in the coating properties. The proposed design of interfacial layers does not relive stress buildup in diamond coating. Also, the proposed two-layered structure is complicated and expensive to deposit.
As such, what is needed is a diamond-coated article which inhibits the diffusion of carbon from the diamond coating. What is additionally needed is a diamond coated composite structure having interfacial layers which resist the diffusion of carbon and do not suffer the aforementioned deficiencies of the prior art.